Low pressure ethylene polymerization and copolymerization is widely practiced commercially. One successful catalyst system for such a polymerization is based on chromium. The basic process is described in U.S. Pat. No. 2,825,721. High density ethylene homopolymers as well as ethylene/1-olefin copolymers of lower density have been commercially produced using chromium catalysts for many years.
The copolymerization of ethylene and other olefins results in the generation of side chains or side groups along the main carbon chain of the polymer. Thus, copolymerization of ethylene and propylene results in methyl side groups while butene-1 or respectively hexene-1, when copolymerized with ethylene over chromium catalysts, results in polymers having ethyl or respectively n-butyl side chains. The number of side chains, their lengths and their distribution in the polymer molecule influence the properties of the polymer. Generally speaking side chains in the polymer tend to disrupt the crystallinity of the polymer and thereby reduce the density.
The price of sufficiently pure 1-olefins has originally caused the use of butene-1 as the major 1-olefin used in the production of modified ethylene polymer resins. Later, when hexene-1 became available at reasonable prices and purities, this olefin was used extensively in the production of such modified ethylene polymers. However, 1-olefins, such as butene and hexene, still are significantly more expensive than ethylene and thereby increase the price of ethylene/1-olefin copolymers produced.
More recently a low pressure polymerization process has been discovered by which a polymer is produced using only ethylene as the monomer and yet resulting in a linear polymer having short side chains. Unlike high pressure (free radical) ethylene polymerization, this process does not cause any significant long chain branching. Such a process for producing linear low density polyethylene using only ethylene as a feedstock is described in U.S. Pat. No. 4,325,839 which issued to Max P. McDaniel. One of the advantages of this new process is that only ethylene and no other olefins are necessary to produce a copolymer with the modifying short side chains.
Although the possibility of making short chain branched, linear ethylene homopolymers using mixed chromium catalysts is an exciting new technology, further development is desirable.